This invention relates to a solid polymer electrolyte for use in electro-chemical devices such as a primary battery, a secondary battery, an electro-chromic display, an electro-chemical sensor, an iontophoresis, a condenser and others.
A solid polymer electrolyte having a crosslink network structure is well known. This crosslink network structure has been formed by using polyethylene glycol or polyether including a molecular weight of smaller than 2,000. The polyethylene glycol has a high ability to dissolve alkaline metal salt, but has a defect that it is liable to crystallize. Further, a solid polymer electrolyte having a structure formed by denaturing and cross-linking the above polyethylene glycol and polyether into ester diacrylate or ester dimethacrylate has the defect that they are inflexible and very brittle. For this reason, it is apt to be broken by an external force to cause short-circuiting, etc. when used in a battery. Especially, the flexibility will be worsened as the molecular weight of polyethylene glycol becomes small. Polyethylene glycols having a molecular weight of smaller than 2,000 have so far been used, and especially those having a molecular weight ranging from 200 to 1,000 have been used in order to avoid crystallizing. Therefore, the defect of poor flexibility has been remarkable.